Automatic repetitive fabrication of workpieces such as by arc or spot welding has been achieved in a variety of ways. In one known instance, a turntable rotating either on a horizontal or vertical axis has a pair of fixtures positioned 180 degrees apart, and both are positioned accurately on the turntable. A worker places unassembled parts or workpieces in one of the fixtures and indexes the system 180 degrees to the work station at which automatic welding may be performed. Servo motors are used for operating the turntable, with precision control means being required to stop the motors to place the workpieces in accurate relation to the welder. While welding is taking place, the worker places a second set of unassembled workpieces on the turntable diametrically opposite the welder. Upon completion of welding the first set and mounting the second set on the turntable, the worker again indexes the equipment 180 degrees to enable removal of the fabricated set and to locate the second set in position to be welded. While such systems perform reasonably efficiently, they are limited in productivity, especially if the product to be fabricated requires complex welding operations or has a large number of pieces. This is due to the fact that the work station ordinarily allows only one person to operate from that position. It further requires that the single person do both tasks of placing workpieces in position and removing finished product from the machine. Additionally, not only must driving be done with precision in relation to the welder, but the entire mechanism must be held to close tolerances, i.e., with minimum "play" of the shafts and bearings in order to place every set of workpieces in the same position each time with respect to an automated, programmed welder.
In another system designed for the same general purpose, a pair of arms have vertical axes spaced apart horizontally, and both are driven in the same horizontal plane. A worker assembles workpieces on a pallet, locates the pallet on a first one of said arms in registration with clamping means thereon, and a servo motor drives the first arm to carry the workpieces through one or more work stations spaced along a peripheral path of travel of the end of the arm. The servo motor stops the arm at each station so that welding can take place at each location. After perhaps 270 degrees of travel, the arm deposits the pallet and partially fabricated product on a fixed-position transfer table and rotates on to its original position to pick up another pallet with unassembled workpieces. The transfer table is motor-driven on a vertical axis. It functions to rotate the pallet 180 degrees in the same horizontal plane for pick up by the second arm in the same relation that the pallet originally had with the first arm. The second arm receives the pallet from the transfer table, and, like the first arm, carries it through one or more work stations positioned along the periphery of the second arm's travel. Upon completion of fabrication, the pallet is transferred to a delivery station either for manual removal or automatic release as was done at the transfer table. The fabricated product may then be removed from the pallet and the pallet reused at the infeed station. This system, while effective, has productivity limitations, requires use of accurately-controlled servo motors and, especially if the welders are robotically controlled arc welders, necessitates that the two arms, the transfer table and the motors for all three be accurately mounted and maintained in relation with one another.